Problem 21 - A Complete Resource Book in Chemistry for JEE Main

Question

In the Castner process of extracting sodium, which of the following reaction occurs at anode (a) \(\mathrm{Na}^{*}+\mathrm{e}^{-} \longrightarrow \mathrm{Na}\) (b) \(4 \mathrm{OH}^{-} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2}+\mathrm{e}^{-}\) (c) \(\mathrm{H}_{2} \longrightarrow 2 \mathrm{H}^{+}+2 \mathrm{e}^{-}\) (d) \(2 \mathrm{Cl} \longrightarrow \mathrm{Cl}_{2}+2 \mathrm{e}\)

Step-by-Step Solution

Answer

The reaction that occurs at the anode in the Castner process is (b) \(4 \mathrm{OH}^{-} \rightarrow 2 \mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2}+\mathrm{e}^{-}\).

Step 1: Understand the Castner Process

The Castner process is used for the extraction of sodium metal by the electrolysis of molten sodium hydroxide. It involves an electrolytic cell with a cathode and an anode.

Step 2: Identification of Anode and Cathode Reactions

In electrolytic cells, reduction occurs at the cathode and oxidation occurs at the anode. We need to identify the reaction that occurs at the anode during the Castner process.

Step 3: Identify the Oxidation Reaction

Oxidation is the loss of electrons. Look for the reaction where electrons are on the product side, indicating a loss. Among the given options, only (b) \[4 \mathrm{OH}^{-} \longrightarrow 2 \mathrm{H}_{2}\mathrm{O}+\mathrm{O}_{2}+\mathrm{e}^{-}\]demonstrates this as water and oxygen are formed, and electrons are released.

Step 4: Confirm the Correct Anodic Reaction

Verify that the chosen reaction fits the process of electrolysis of molten sodium hydroxide. The reaction forming water, oxygen, and releasing electrons matches the conditions at the anode in the given electrolysis.

Key Concepts

Electrolysis of Sodium Hydroxide

The electrolysis of sodium hydroxide is an essential industrial process used for the extraction of sodium. Sodium, being a highly reactive metal, is typically extracted through a specialized electrochemical method called the Castner process. Here's how it works:

• In this process, sodium hydroxide (\( NaOH \)) is melted to become a liquid electrolyte, which allows ions to move freely.
• An electrolytic cell setup is used, consisting of two electrodes: an anode and a cathode.
• When an electric current passes through the molten sodium hydroxide, various reactions occur at the anode and cathode.

The main goal of the electrolysis of sodium hydroxide is to generate pure sodium metal at the cathode. The process exploits the movement of ions and involves separating different elements based on their reactivity and anticipated reactions at the electrodes. Importantly, understanding the nature of the electrolyte and the reactions at each electrode is crucial for successful extraction.

Oxidation Reaction

An oxidation reaction is one half of a redox (reduction-oxidation) process and is characterized by the loss of electrons by a molecule, atom, or ion. In any electrolysis:

• The site of oxidation is the anode, where substances lose electrons.
• These lost electrons travel through the external circuit towards the cathode, where a reduction reaction occurs.

In the context of the electrolysis of sodium hydroxide, the specific oxidation reaction can be identified by examining which reactions involve the release of electrons.For instance, in the options given in the original problem, reaction (b) - \(4 \mathrm{OH}^{-} \rightarrow 2 \mathrm{H}_{2} \mathrm{O} + \mathrm{O}_{2} + 4 \mathrm{e}^{-}\) - clearly shows oxidation because the hydroxide ions (\( \mathrm{OH}^{-} \)) lose electrons and form water and oxygen. Recognizing the flow of electrons, from anode to cathode, is a fundamental aspect of understanding oxidation reactions in electrolysis.

Anodic Reaction in Electrolysis

The anodic reaction in electrolysis is the oxidative half-reaction occurring at the anode. This involves the loss of electrons, which are then carried through the circuit to the cathode. Consequently, at the anode:

• Substances undergo oxidation, releasing electrons.
• For the electrolyte of sodium hydroxide, the anode reaction is key to the successful extraction of sodium metal.

In the Castner process, the anodic reaction is specifically \(4 \mathrm{OH}^{-} \rightarrow 2 \mathrm{H}_{2} \mathrm{O} + \mathrm{O}_{2} + 4 \mathrm{e}^{-}\). Here, hydroxide ions (\( \mathrm{OH}^{-} \)) lose electrons, leading to the formation of water (\( \mathrm{H}_{2} \mathrm{O} \)) and oxygen (\( \mathrm{O}_{2} \)). This transformation is pivotal as it not only provides electrons necessary for reduction at the cathode but also sustains the ionic flow, which is essential for any electrolytic process.Understanding the anodic reaction is crucial, as it helps predict the products and also ensures proper control over the electrolysis process.